Amplifiers are used in many applications to amplify an incoming signal into an amplified signal. For example, amplifiers are often used in communication systems to boost the level of an incoming signal and sometimes to shape the signal in some desired way. Certain communication systems transmit data with a clock embedded in a data stream, rather than as a separate signal. When the data stream is received, a clock and data recovery circuit (CDR) recovers the embedded clock and retimes the received data to the recovered clock. Oftentimes, a CDR is implemented in an integrated circuit along with additional components, such as a limit amplifier (LA) and other such components. The LA may receive a voltage signal from a transimpedance amplifier (TIA) or other amplifier, which amplifies an incoming converted optical signal.
The function of the limit amplifier is to produce a consistent waveform from the TIA output which can be used by the CDR, regardless of incoming optical energy. In addition to amplifying the input signal, the LA may provide an adjustable slicing level to compensate for an asymmetric noise characteristic present in the incoming data. A slicing level is the threshold voltage at which an incoming signal is determined to be either a “1” bit or a “0” bit. At low levels of optical energy (e.g., corresponding to a zero bit level, for example), the noise current is low. At higher levels of optical energy (corresponding to a one bit), the noise current may be higher. An optimal slice level for an amplifier in a receive path can enhance receiver performance significantly, especially in long-haul applications. Thus an offset is typically inserted into the receive path, either at an input of an amplifier or at an output thereof.
The offset voltage, referred to as a slice voltage in certain applications, is often applied to be summed with the input of an amplifier. This offset voltage may be used to compensate for a slice level at which an amplified signal is to be sampled. Accordingly, the output of an amplifier may correspond to the sum of the input voltage and offset voltage, multiplied by the gain of the amplifier.
Referring now to FIG. 1A, shown is a block diagram of a limit amplifier 10 that receives an incoming voltage, Vin, along with a slice voltage, Vslice, and provides an amplified output signal, Vout. The output voltage of amplifier 10 may be described in accordance with the following equation:Vout=(Vin+Vslice)×GAINLA  (Eq. 1)where GAINLA is the gain of limit amplifier 10. While limit amplifier 10 is shown in FIG. 1A as a differential amplifier, a single-ended implementation is also possible. Further, while the slice voltage is shown as a single-ended input, this voltage may also be a differential signal.
Optical signals are single-ended in nature. That is, a logic one value provides light, while a logic zero signal is dark. There is asymmetry in “1” and “0” signals when they are converted into the electrical domain, creating an asymmetrical data eye pattern. FIG. 1B is an eye diagram of a typical incoming data signal at an input of a LA and an amplified version appearing at the output of the LA. As shown in FIG. 1B, a data eye 20 corresponding to the incoming data is formed by superimposing waveforms of multiple data signals.
As shown in FIG. 1B, an output data eye 30 corresponding to the signal at the output of a LA is clipped at the top and bottom via the limit amplifier. Further shown in FIG. 1B is a slice voltage, Vslice, which corresponds to a widest opening of the eye pattern. The slice voltage may be adjusted depending upon the incoming signal to maintain the slice level at a widest portion of the eye pattern.
In the real world, data signals forming a data eye have transitions with varied rise times and fall times and may also exhibit different voltage levels and shapes. Thus a slicing level may be selected or controlled to obtain an output data eye with its widest opening and a relatively symmetric margin between the positive swing voltage and the negative swing voltage. To achieve the desired output signal, an introduction of an intentional offset may be effected, to optimize noise margin and hence achieve a lower bit error rate (BER).
Typically, this offset or slice voltage is proportional to the magnitude of the input voltage. A limit amplifier is a highly non-linear device, as the purpose of a limit amplifier when used, for example, in an optical system is to produce a clipped output such that a downstream device (e.g., a CDR) can easily process the data signal. Accordingly, the gain of the limit amplifier is not constant; for a small input voltage the gain is large, while for a large input voltage the gain is small.
It is desirable to generate a slice voltage that is at a low noise level, as noise in this slice voltage is amplified in the same manner as the input voltage. Typically, slice control is implemented by adding a slice amplifier stage to a signal amplifier signal stage in the limit amplifier. Both amplifier stages are generally controlled using independent fixed bias currents. Noise present in the output signal of the amplifier may originate in various sources, including circuitry that provides the input voltage, for example, a photodiode and a transimpedance amplifier that convert incoming optical energy into electrical signals. Furthermore, noise can be generated by the components of the signal amplifier stage as well as the slice amplifier stage. Still further, circuitry that generates a control voltage that is used to generate the slice voltage can also lead to noise.
In order to reduce noise contribution from the slice amplifier stage, typically the transconductance (gm) of the devices in the slice amplifier stage are much smaller than the transconductance of the devices in the signal amplifier stage. Because the bias currents remain fixed, so too does this transconductance ratio remain fixed.
Furthermore, because the signal swing range of a slice voltage is limited by the linear range of the slice amplifier stage and the available voltage swing of the control voltage, the achievable slice range is compromised, resulting in a less than satisfactory slice control range. For example, it is common for a slice range to be limited to approximately 20% of the incoming signal strength. In actual terms, this slice level is often limited to 100 millivolts (mV) or less in an optical receiver.
A need thus exists for an improved manner of controlling an offset signal range such as providing an extended slice adjust range.